September 15, 2017. High above the Saturnian clouds, a spacecraft prepares for its final journey. That spacecraft is Cassini, and it is on a journey from which it will never return. It enters the atmosphere and beams back humanity’s first glimpse of what lies under the marbled cloud tops of Saturn. The deeper it goes, the more effort it takes to point the antenna at Earth, and before long, the atmosphere begins to tear the probe apart. Signal is lost shortly after, and ultimately, the tumbling, lonely spacecraft becomes a meteor in the skies of the planet it has studied for so long. After two decades of exploration, Cassini has become one with Saturn.
Three years after the mission ended, Cassini remains one of the most ambitious and successful projects in the history of space exploration. Even today, discoveries are still being made from the vast amount of data returned by the probe, and these findings have single-handedly shaped our understanding of Saturn and the outer Solar System in general. Cassini unlocked some of the secrets of the faraway ringed planet, but its lingering legacy has also brought about new questions that will keep scientists busy for decades.
Humans have always been fascinated with Saturn, the farthest of the planets visible with the naked eye. Lying beyond the orbit of Jupiter, it remained a simple, non-twinkling dot in the sky for much of human history, until Galileo saw its magnificent rings for the very first time, mistakenly thinking they were moons. Christiaan Huygens would later prove that they were in fact rings, and Giovanni Domenico Cassini found that the rings possessed intricate structures. The two astronomers would become the namesakes of the spacecraft that would explore the ringed planet more than 300 years later.
Three years after the mission ended, Cassini remains one of the most ambitious and successful projects in the history of space exploration. Even today, discoveries are still being made from the vast amount of data returned by the probe, and these findings have single-handedly shaped our understanding of Saturn and the outer Solar System in general. Cassini unlocked some of the secrets of the faraway ringed planet, but its lingering legacy has also brought about new questions that will keep scientists busy for decades.
Humans have always been fascinated with Saturn, the farthest of the planets visible with the naked eye. Lying beyond the orbit of Jupiter, it remained a simple, non-twinkling dot in the sky for much of human history, until Galileo saw its magnificent rings for the very first time, mistakenly thinking they were moons. Christiaan Huygens would later prove that they were in fact rings, and Giovanni Domenico Cassini found that the rings possessed intricate structures. The two astronomers would become the namesakes of the spacecraft that would explore the ringed planet more than 300 years later.
The first time humanity got an up-close look at Saturn was in 1979, when the Pioneer 11 spacecraft made a brief fly-by of the planet. Voyager 1 flew past Saturn and its moon Titan in 1980 before continuing to interstellar space, and its sister probe Voyager 2 did the same a year later on its “Grand Tour” of the four outer planets of the Solar System. Although much was learned from these early missions, their flybys only lasted for a few hours, and so data returned was limited. To truly observe Saturn over a long period, a spacecraft would need to enter Saturnian orbit.
The Cassini mission was first conceived in 1982, when European and American scientists proposed to their respective space agencies for a Saturn orbiter mission, with possible collaboration with each other. After six years of review, the Cassini-Huygens project was finally approved by the European Space Agency and NASA, with each providing funding, hardware, and technical expertise. The final design for the spacecraft consisted of two parts: Cassini, the main orbiter which studied Saturn itself, and the Huygens lander, which explored Saturn’s largest moon, Titan. Cassini turned out to be one of the largest and heaviest spacecraft ever launched at 5600 kilograms, as well as one of the most complex since it used around 14 kilometres of wiring.
Cassini-Huygens began its odyssey to the outer solar system from Cape Canaveral, Florida on October 15, 1997. From there, it performed two flybys of Venus and one of Earth to guide its trajectory away from the inner Solar System. These fly-bys were necessary because missions to Saturnian orbit are extremely energy-intensive, and the gravitational wells of planets are the only way to meet this energy demand for now. One final flyby of Jupiter in 2000 sent Cassini on a course that would take it to Saturn, and after six and a half years of travelling, the spacecraft entered orbit in early 2004.
Orbit insertion was only the beginning of Cassini’s mission, which saw it explore everything from the fabled rings to the numerous moons of Saturn. One of the first major events at the planet occurred on Christmas Day of 2004 when the European Huygens lander detached from Cassini, and less than a month later, on January 14, 2005, it performed the first successful landing in the outer solar system. The incredible success of the mission in just its first few years, combined with the spacecraft’s optimal state of health provided Cassini with two extensions beyond its original three-year plan. As a result, scientists gained an opportunity to observe Saturn for an unprecedented amount of time. Unfortunately, time was not unlimited, and with fuel reserves running low, concerns emerged over the biological contamination that could occur if Cassini collided into Saturn's moons, some of which are candidates for alien life. Therefore, mission planners decided to intentionally burn Cassini up in the Saturnian atmosphere.
However, simply deorbiting the spacecraft would not be a dignified end to such an important mission, so a daring plan known as Cassini’s Grand Finale was conceived. The plan involved 22 dives between Saturn and its rings, and when the last of these maneuvers was completed, all that was left was to wait. At last, on September 15, 2017, Cassini’s 20-year long mission came to an end.
Cassini’s impact on space exploration has been immeasurable in many ways. During its mission, it helped revolutionize our understanding of Saturn itself, as well as its magnificent rings and many moons. Its many mission extensions also helped, as the 13 years Cassini spent at Saturn nearly encompassed half of the planet’s orbital period. This allowed scientists to track Saturn’s seasons over many years and gain a better sense of the weather patterns within its atmosphere. However, the value of Cassini was not just limited to new discoveries. The mission also confirmed many observations conducted by the Voyager and Pioneer probes that preceded it, which further added to the scientific yield.
In the closing phases of its mission, Cassini found that the rings of Saturn were relatively young, and may have formed as recently as when dinosaurs walked on the Earth, showing us how fortunate we are to witness this wonder. It also saw that the rings were made of intricate structures carved out by its many moons, and discovered clumps of material where potential new moons were starting to form, just like in the early days of the Solar System. In a sense, Cassini showed us that the Saturnian system was indeed a mini-Solar system.
Perhaps, most importantly, Cassini gave us hope that life may exist past our planet, as data collected revealed that two of Saturn’s moons, Titan and Enceladus, are some of the most promising candidates for extraterrestrial life. Titan is the only moon in the Solar System that has an atmosphere, and its geology is very similar to Earth’s, albeit with a different composition. As Huygens descended onto its surface, it saw mountains of water ice and a thick methane atmosphere blocking out the sun. Cassini even found giant lakes of liquid methane on its surface. Interestingly enough, the picture of Titan that Cassini-Huygens painted has led many scientists to believe that the moon may be a window into what our planet looked like in its early days, around the time when life first emerged here.
Enceladus, on the other hand, is an icy world devoid of an atmosphere, but Cassini discovered giant plumes ejecting salty water into space, hinting at the existence of a subsurface ocean. In one of its more daring adventures, the spacecraft even flew through these plumes, which gave scientists a sampling of what the undersea environment may be like. Whether life could be found in this ocean is uncertain, but it, along with the discoveries made on Titan, can be considered one of the most important scientific observations made in the quest for alien life.
No spacecraft has visited Saturn since Cassini entered its atmosphere on that day in September 2017. However, new missions are already being planned to solve some of the many new questions raised by Cassini-Huygens. For instance, NASA recently approved Dragonfly, a mission arguably as ambitious as Cassini itself, which, by taking advantage of Titan’s thick atmosphere, will fly around the moon to investigate its various regions. The “flying rover” aims to study the moon’s fascinating chemistry, which will allow scientists to picture the early Earth, and to find out whether Titan could support carbon-based or non-carbon-based life.
Another ESA/NASA collaboration, the Titan Saturn System Mission (TSSM), consisting of an orbiter, a Titan lander and a balloon, was proposed in 2009, but a Jupiter mission was given priority over it. However, it is still possible that this mission could go ahead, and it will continue to be assessed for future development. Either way, the future of Saturn exploration looks bright, and there is still much more to be discovered.
Cassini sent back a massive amount of data from the ringed planet, and even now, we have only begun to scratch the surface of what lies in the Saturnian system. It is entirely possible, and even probable, that new discoveries will continue to be made from the data collected by Cassini for decades to come, and future missions will only boost our knowledge of the planet that has captured our imagination for centuries. It is a fitting legacy for a lonely spacecraft whose 20 year-long odyssey shattered the expectations of everyone on Earth. Cassini has opened our eyes to the far reaches of the outer Solar System, and there is no turning back from the sea of secrets that is waiting to be revealed.
Sources & Further Reading
Cassini mission overview
Cassini's ring discoveries